This invention relates to arc evaporation processes and, in particular, to such processes and apparatus for arc evaporating large area targets.
The arc evaporation process is used in arc coating systems such as those disclosed in U.S. Pat. Nos. 3,625,848 and 3,836,451 to Alvin A. Snapper and U.S. Pat. Nos. 3,783,231 and 3,793,197 to L. Sablev, et. al, all of these patents being incorporated herein by reference. These systems are characterized by the use of arc guns to effect high deposition rates and other advantageous features. The arc itself involves currents of 60 amperes or more concentrated into a cathode spot so small that current densities are 10.sup.3 to 10.sup.6 amperes per square inch, the mean energy of the particles constituting the arc being typically 20-100 electron volts. The voltages are 15 to 45 volts. Thus, power densities at the tiny cathode spot are in the order of megawatts/inch.sup.2. The cathode spot migrates about the target surface in a random, jerky motion where the target surface under the cathode spot flash evaporates from the intense heat. It is the evaporated target material which deposits as the coating on a substrate.
Because of this random movement of the cathode spot, targets of relatively small dimensions can be relatively uniformally eroded while coating a relatively small substrate and thus efficient target utilization is effected. However, when the substrate size is increased, a difficulty arises if one simply attempts to employ a target size greater than about 20 square inches, in that uniform erosion of the target no longer occurs, thus resulting in poor target utilization. Rectangular targets are especially poor in this regard. Thus, when large substrates are involved, multiple small sources have been heretofore employed with correspondingly larger total target area where each source constitutes an arc gun typically requiring about 60 amperes to generate at least one arc spot. The substrate heating increases proportionately to the number of spots. Thus, as the load is expanded (including the size of the substrate to be coated), there is a tendency to damage the substrate by overheating due to the large number of guns employed. Thus, although the large number of guns do provide freedom of adjustment, this approach tends to be costly both from the point of view of power supply and maintenance needs. Accordingly, a more practical approach is needed for the commercial coating of large surface areas or large racks of smaller parts.